Driven by a strong demand for portable electronic devices, non-volatile memory represents an important and rapidly growing sector of today""s semiconductor memory market. Polysilicon floating gate memory devices conventionally have held the largest market share of non-volatile memory devices. In today""s rapidly emerging non-volatile memory device sector, non-volatile floating gate memory devices have been fabricated by embedding silicon nanoparticles within the gate oxide of metal-oxide semiconductor field effect transistors (MOSFETs). Nanoparticles are so named because they include particle diameters on the nanometer scale. It is believed that silicon nanoparticle floating gate memory devices outperform conventional floating gate memory devices with faster read and write times, higher reliability, and lower power dissipation. The memory operation of nanoparticle field effect transistors depends on charge storage, similar to conventional non-volatile memory devices. In a nanoparticle non-volatile memory device, however, charge is not stored on a continuous floating gate polysilicon layer as in the conventional technology, but instead on a layer of discrete, preferably crystalline silicon nanoparticles which may alternatively be referred to as nanocrystals or quantum dots.
In these nanoparticle floating gate memory devices, the nanoparticles that act as charge storage elements are located within the gate oxide of a MOSFET. Injecting charge into the nanoparticles by tunneling from the channel alters the threshold voltage of the transistor. A normal write/read/erase cycle includes information being written by injecting charge from the channel into the nanoparticles, reading by measuring the subthreshold current-voltage characteristics, and erasing by removing charge from the particles to the channel. A single electron stored on each nanoparticle in an array with a nanoparticle density of 3-10xc3x971011/cm2 results in a threshold voltage shift of 0.3-0.5 volts that is easily detected at room temperature. Generally speaking, as compared to conventional stacked-gate non-volatile memory devices, nanoparticle charge-storage offers several potential advantages, such as: (1) simple, low cost device fabrication since a dual-polysilicon process is not required; (2) superior retention characteristics resulting from Coulomb blockade and quantum confinement effects, enabling the use of thinner tunnel oxides and lower operating voltages; (3) improved anti-punchthrough performance due to the absence of drain-to-floating gate coupling thereby reducing drain induced punchthrough, allowing higher drain voltages during readout, shorter channel lengths and consequently a smaller cell area; and (4) excellent immunity to stress induced leakage current (SILC) and defects, due to the distributed nature of the charge storage in the nanocrystal layer. Even if a significant fraction of the individual nanocrystals that form the floating gate, are shorted to the channel/substrate, the non-volatile memory device remains functional because the non-shorted nanocrystals continue to store sufficient charge. The switching speed of devices made of nanocrystal ensembles, however, is potentially limited by a distribution in charge transit times, charging voltages, and threshold shifts resulting from various shortcomings of the nanoparticle layer, such as the nanoparticle size and size distribution, nanoparticle density, layer planarity and uniformity, and nanoparticle-to-nanoparticle interaction, i.e., lateral conduction.
Thus, there is a demonstrated need in the art for a layer of nanoparticles of uniform size distribution and density. Similarly, there is a demonstrated need for fabricating silicon or silicon-compatible nanocrystals with controlled size distributions and oxide thicknesses that can be deposited on a substrate in a uniform and co-planar manner. It is also desirable to fabricate the layer of nanocrystals using a process sequence that is simple, reliable, low cost, easily controlled, repeatable, and free of contamination. Previous attempts at producing a layer of nanocrystals suitable for use in a field-effect transistor or other non-volatile memory devices, include the shortcomings of uncontrolled particle sizes, non-uniformity of particle deposition, high contamination levels, low density of the particle material, non-uniform density of the particles within the nanoparticle layer, and unpredictable planarity of the nanoparticle layer. Such irregular and unpredictable nanocrystal layers result in poor-performing or non-functional devices.
In conclusion, in order to produce non-volatile memory devices with faster read and write times, higher reliability and lower power dissipation, it is desirable to produce nanocrystal floating-gate non-volatile memory devices using a simple, low cost fabrication process which provides a layer of nanocrystals which forms a monolayer of nanocrystals of uniform density and particle size.
To address these and other needs and in view of its purposes, the present invention provides a process for forming a stratum of semiconductor or metal particles having sizes in the nanometer range and suitable for application as the floating gate in a non-volatile memory device. The stratum is composed of particles having a tightly controlled range of particle sizes. The process includes decomposing a source of semiconductor or metal material to form an aerosol of nanoparticles, then sintering or heating the nanoparticles of the aerosol to convert the particles to crystalline material. In an exemplary embodiment, the process may include quenching the nanoparticle aerosol to minimize uncontrolled coagulation and to further control particle size. In an exemplary embodiment, the majority of nanoparticles are single crystalline material. The densified nanoparticles are compacted and include a density which approaches the bulk density of the material of which they are formed. The densified nanoparticles may optionally be classified by size, and particles outside the range of desired sizes are removed from the aerosol stream.
The particles are preferably coated with a substantially continuous insulator coating to produce particles having crystalline cores and a substantially continuous insulating shell. The particles are then deposited onto a substrate surface using thermophoretic or other means. The deposited, insulator-coated particles form a stratum on the substrate surface, and in one embodiment, may be utilized as a floating gate in a non-volatile memory device. The particles of the stratum are electrically isolated from one another.